(a) Field of the Invention
The invention relates to a motor control circuit and, more particularly, to a motor control circuit capable of diminishing back electro-motive force (emf) to reduce switching noise.
(b) Description of the Related Art
FIG. 1 is a schematic diagram illustrating a conventional bridge circuit, which includes a first drive unit having an opposite pair of transistors Q1 and Q4 and a second drive unit having an opposite pair of transistors Q2 and Q3. As a pulse width modulation (PWM) signal generating unit (not shown) generates a square wave shown in FIG. 2, it is first fed to the bridge circuit to make the first drive unit enabled, allowing current to flow through the motor coil 10 in the direction plotted in solid line, as shown in FIG. 3. Then, the phase angle of the input square wave is changed to make the second drive unit enabled and the first drive unit disabled, allowing the current to flow through the motor coil 10 in another direction as plotted in dash line. By determining which pair of transistors is enabled, the current can be made to flow in either of the two directions through the motor. Because permanent-magnet motors reverse their direction of turn when the current flow is reversed, this bridge circuit allows bi-directional control of the motor.
However, in conventional PWM control as illustrated in FIG. 2, since the switching time from “On time” period to “Off time” period (pulse level H to 0), or from “Off time to “On time” (pulse level 0 to H), is very short, such abrupt change in pulse level may result in a considerable back electro-motive force that highly influences the operating voltage and current in a fan motor, as shown in FIG. 4, and the switching noise of the driving circuit is greatly increased as a result.